Well...Not necessarily. GT adults tend to have GT denial about themselves, too, not just about their kids. We tend to think of ourselves as more average/less GT than we are. In fact, that can be one of the reasons we don't realize just how bright our kids are: "He's like me, and I was probably MG, so he must be MG, too." But if Mom is NOT MG, but is actually HG+, then the whole train of logic is off.
I wouldn't toss out the idea that there's some genetic component of GTness just yet...
My DW intially said that she is not GT.
I questioned her closely about her education.
Turns out she skipped 1st grade. She could not recall her first grade teacher, but could recall all the others. She was the top child in her 8th grade class and won some district wide awards. She has nearly total recall of anything she hears.
I think she has some sort of deficiency when it comes to reading. She is smoking fast on excel, etc, and can usually learn and max out on a videogame in one night, but she reads 1 page every 5 minutes. She does not have total recall for what she reads, but can look at a page of numbers and pick out the discrepancies AND can tell you if something changed if she has seen that page before.
